The invention relates to a current overload tripping device operating to provide a time-delayed leading tripping action when there is an uneven loading of the phases of a multiphase circuit. Such a device is especially useful for motor protection. Generally, a transformer current pickup member is assigned to each phase of the multiphase circuit with the current pickup members being connected in series with each other and with an electromechanical converter. A current overload tripping device of this type is described in DE-C-674 539. The leading tripping action is caused by the fact that an uneven loading of the phases causes a difference current which is fed to a tripping member. If the phases are loaded evenly, however, no difference current occurs. A summing current transformer is used to determine the phase currents. The core of the summing current transformer includes several windings through which the phase currents flow, as well as series-connected secondary windings. Such an arrangement requires a relatively large amount of space for implementation.
In this connection, it is an object of the present invention to simplify the design of a current overload tripping device having a leading tripping action and to thereby permit the use of the leading characteristic in relatively small switch gear structures having a relatively limited amount of space available for installing such tripping devices.
According to the invention, the space problem is obviated by the provision that a winding surrounding the magnet s yoke of an electromagnetic tripping device of each phase serves as the current pickup member, and a temperature/time-dependent arrangement for delivering a time-delayed tripping signal serves as the electromechanical converter. Accordingly, a separate summing current transformer is not required. The windings about the magnet yokes can be accommodated practically without any additional space requirements in the switch gear structure since they each require only one or a few turns. By evaluating the difference current in a temperature/time-dependent device, an asymmetry does not necessarily cause an immediate tripping action and, thereby, an interruption of operation. Only the presence of an asymmetry over a certain time interval leads to tripping. For instance, the electromechanical converter can include, for this purpose, a break contact for the exciter winding of the magnetic drive of a contactor. A break contact, for instance, would be suited for actuating the auxiliary tripping device of a protective switch. For normal overload protection, ordinary thermal tripping devices or relays can be used which operate without lead and are, therefore, of simple design. If required, a current overload tripping device according to the invention can be added which requires no mechanical connection of any kind with the thermal tripping device and need not even be arranged in its proximity. This results in a clearcut design which utilizes the available space of compact switch gear structures in an efficient and economical manner.
Moreover, only a very small amount of auxiliary energy is required to cause a tripping signal. Therefore, the current pickup members do not have to meet stringent requirements Pursuant to one embodiment of the present invention, the temperature/time-dependent arrangement can comprise, for example, a bimetallic element such as is also used for normal current overload tripping devices. Pursuant to another embodiment of the invention, the temperature/time-dependent arrangement can comprise heatable elements made from a shape-memory alloy. Bimetallic elements as elements of a shape-memory alloy can be provided in a directly or indirectly heated arrangement.
A switch can be inserted into the series circuit of the current pickup members and the electromechanical converter. This switch permits an optional operation, as a function of switch position, of a switch gear with or without a leading tripping action.
The invention will be described in greater detail in the following detailed description making reference to the exemplary embodiment illustrated in the figure.